SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a substrate such as silicon. APD cells vary in dimension from several micron to 100 microns depending on the mask used, and can have a typical density of up to 3000 microcells/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high speed telecommunications using optical fibres, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 100 to >1000V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 100->200 V in silicon, depending on the doping profile in the junction). Silicon Photomultipliers or SiPMs can achieve a gain of 105 to 106 by using Geiger mode APDs also known as Single Photon Avalanche Diodes (SPADs) which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level. The current generated by an avalanche event must be quenched by an appropriate current limited scheme so that the device can recover and reset after an avalanche event. SiPM sensors have lower operating voltages than APDs and have a breakdown voltage from 10-100 Volts.
Silicon Photomultipliers typically include a matrix of major and minor bus lines. The minor bus lines connect directly to the microcells. The minor bus lines are then joined to bond pads by a major bus line. The minor bus lines are loaded with the inductance, capacitance and resistance of the microcells. The major bus lines are then loaded with the inductance, capacitance and resistance of the minor bus lines. The rise time, delay and recovery time of signal from a microcell on the SiPM will therefore depend strongly on its position in the SiPM. The variation in rise time and delay across the array will give rise to increased jitter and therefore increased coincidence resolving time (CRT) or timing jitter.
There is therefore a need to provide a semiconductor photomultiplier which addresses at least some of the drawbacks of the prior art.